Future wireless systems require a more effective utilization of the radio frequency spectrum in order to increase the data rate achievable within a given transmission bandwidth. This can be accomplished by employing multiple transmit and receive antennas combined with signal processing. A number of recently developed techniques and emerging standards are based on employing multiple antennas at a base station to improve the reliability of data communication over wireless media without compromising the effective data rate of the wireless systems. So called space-time block-codes (STBCs) are used to this end.
Specifically, recent advances in wireless communications have demonstrated that by jointly encoding symbols over time and transmit antennas at a base station one can obtain reliability (diversity) benefits as well as increases in the effective data rate from the base station to each cellular user per unit of bandwidth. These multiplexing (throughput) gain and diversity benefits depend on the space-time coding techniques employed at the base station. The multiplexing gains and diversity benefits are also inherently dependent on the number of transmit and receive antennas in the system being deployed, in the sense that they are fundamentally limited by the multiplexing-diversity trade-offs curves that are dictated by the number of transmit and the number of receive antennas in the system.
For high data rates and wideband transmission, the use of OFDM makes the equalizer unnecessary. With multilevel modems, coded modulation systems can easily be designed by means of an outer binary code, e.g., a convolutional code, and an interleaver in a so called bit-interleaved coded modulation (BICM) system.
In many emerging and future radio networks, the data for any particular cell user may be available to multiple base stations. Joint signaling from multiple base stations can readily extend the range/coverage of the transmission. Furthermore, viewing each of the base stations with data for a particular user as an element (or a group of elements in the case that multiple transmit antennas are present at each base station) of a virtual antenna array suggests using cooperative signal encoding schemes across these base stations to provide diversity benefits to the desired user. Since the encoded signals, however, are transmitted by spatially dispersed base-stations, they arrive at the receiver with distinct relative delays with one another, i.e., asynchronously. Although these relative delays can, in principle, be estimated at the receiver, they are not known (and thus cannot be adjusted for) at the transmitting base stations, unless there is relative-delay information feedback from the receiver to the transmitting base stations.
A large collection of STBCs have been proposed in recent years as a means of providing diversity and/or multiplexing benefits by exploiting multiple transmit antennas in the forward link of cellular systems. Of interest is the actual symbol rate of the STBC scheme, R, which is equal to k/t (i.e., the ratio of k over t). Full rate STBCs are STBCs whose rate R equals 1 symbol per channel use. Another important attribute of a STBC is its decoding complexity. Although the decoding complexity of the optimal decoder for arbitrary STBCs is exponential in the number k of jointly encoded symbols, there exist designs with much lower complexity. One such attractive class of designs, referred to as orthogonal space-time codes (OSTBCs), can provide full diversity while their optimal decoding decouples to (linear processing followed by) symbol-by-symbol decoding. Full rate OSTBCs exist only for a two transmit-antenna system. For three or more antennas, the rate cannot exceed 3/4 symbols/per channel use. This rate is achievable for N=3 and N=4 antennas. As a result, although the imposed orthogonality constraint yields simple decoding structures, it places restrictions in the multiplexing gains (and thus the spectral efficiencies and throughput) that can be provided by such schemes.
Many MIMO/OFDM systems exploit large-size QAM constellations and BICM/ID and have an inner MIMO detector block of high complexity.
A number of systems deployed for broadcasting common audio/video information from several base stations are exploiting coded OFDM transmission under the umbrella of the single frequency network concept. These systems employ a common coded OFDM based transmission from each of the broadcasting base-stations. The OFDM based transmission allows asynchronous reception of the multitude of signals and provides increased coverage. However, as all base-stations transmit the same coded version of the information-bearing signal, SFN (single frequency network) systems do not provide in general full transmit base-station diversity with full coding gains (some form of this diversity is available in the form of multi-path diversity, although limited since it is not coordinated). A scheme with an inner modified orthogonal STBC can be viewed as a method that provides the OFDM based benefits of a single frequency network while at the same time allowing the full transmit base-station diversity and frequency diversity to be harvested from the system by using distinct coordinated transmissions from distinct base stations together with bit-interleaved coded modulation.
A class of schemes that can provide large spectral-efficiencies and reliable transmission includes space-time bit-interleaved coded modulation systems with OFDM. These systems can provide spatial (transmit and receive antenna) diversity, frequency diversity and can cope with asynchronous transmission. Furthermore, by modifying the binary convolutional code to a block with rate compatible punctured convolutional codes, a flexible UEP system can be achieved. One drawback associated with such systems is that the near-optimum receiver can be quite complex (computation intensive). The necessary joint demapper unit (inner MAP or MaxLogMAP decoder) grows in complexity exponentially with the product of the number of transmit antennas and the number of bits per modem constellation point. As an example with 16 QAM (4 bits/symbol) and 4 transmit antennas, the complexity of the calculations in the inner decoder is proportional to 24×4=216.
It is well known that the Gray mapper for the QAM constellations is a good choice for the noniterative decoder but not for the iterative decoder.